1. Field of the Invention
The present invention relates to a charged particle beam exposure method, and particularly to a method of controlling beam deflection in a charged particle beam exposure system that employs a continuously moving stage technique and a double deflection technique.
2. Description of the Related Art
To improve the throughput of a charged particle beam exposure system, one attractive method is to continuously move a stage while exposing a sample on the stage to beams using the double deflection technique to draw required patterns on the sample.
According to this method, a sample to be placed on the continuously moving stage and exposed to beams is divided into a plurality of main fields (each, for example, 2 mm square), and each of the main fields is divided into subfields (each, for example, 100 .mu.m square). A major deflector deflects a charged particle beam to a predetermined position of the main field. The major deflector mainly comprises an electromagnetic deflecting coil having a wide deflection width and a relatively slow deflection speed. A minor deflector forms a pattern on each exposure section in each subfield. The minor deflector mainly comprises an electrostatic deflector having a narrow deflection width and a relatively high deflection speed.
Since the stage moves continuously during exposure, the major deflector and minor deflector must be controlled according to movements of the moving stage to correctly deflect a beam at any time. This is very important in order to draw predetermined patterns at predetermined positions.
In adjusting a deflector within a certain range according to an instruction signal, it is necessary to consider a settling time of the deflector. Due to actions of coils, amplifiers, etc., of the deflector, the deflector needs some settling time to stabilize itself at a specified position.
The minor deflector generally exposes a sample at a speed of about 100 nsec, and therefore, finishes the exposure of each subfield of the sample in 150 nsec (about 6 MHz). A settling time of the minor deflector is said to be 30 to 50 nsec. This amount of settling time does not cause a severe problem in exposure.
On the other hand, the major deflector for deflecting a beam toward a main field of the sample needs a settling time of about 20 .mu.sec for a jump distance of 100 .mu.m because the major deflector employs electromagnetic deflection. For a jump distance of 2 mm, the major deflector needs a settling time of at least 1 msec. This long settling time is due to inductance of the coil and a magnetic field produced by an eddy current flowing through lens conductors, and causes a serious problem in adjusting the major deflector.
The composite of FIGS. 1A to 1C (hereafter, "FIG. 1") shows an example of the conventional charged particle beam exposure system employing the continuously moving stage technique and double deflection technique.
In FIG. 1, a major deflector buffer memory 1 stores positions (coordinates) of a major deflector for subfields S1 to Sn. As shown in FIG. 2, the center of each main field M forms an original point Cm of a coordinate system for pattern data of the subfields S1 to Sn contained in the main field M. A subfield buffer memory 2 stores the pattern data for the subfields S1 to Sn.
To draw patterns in a selected main field M, a major deflector position (coordinate) X1 of a selected subfield in the main field M is read out of the memory 1. ("Main" and "major" are used synonymously herein.) The coordinate X1 is design data that represents, for example, the center of the selected subfield. The read coordinate X1 is stored in a major deflector position memory 3.
Pattern data of the selected subfield (one of the subfields S1 to Sn) of the main field M are read out of a subfield coordinate section of the memory 2. At this time, a subfield pattern generator 4 is not yet activated.
At the instant when the major deflector coordinate X1 of the subfield is stored in the major deflector position memory 3, a laser interferometer 6 reads a position (coordinate) of a stage 5. The read coordinate is passed through a laser counter 7 and stored in a stage read register 8. A subtracter 10 computes a difference between the coordinate stored in the stage read register 8 and a target position on the stage 5 where the center of the main field M must exist, stored in a stage target register 9. An adder 11 adds the difference computed by the subtracter 10 to the major deflector coordinate X1 of the selected subfield, and provides a distance X1 (a major deflector shifting vector) for shifting the major deflector along, for example, an axis X. The vector X1 is stored in a major deflector shifting coordinate memory 12.
The minor deflector is also corrected according to movements of the stage. Since the stage continuously moves, the direction and width of deflection of each deflector must always be checked and controlled. To efficiently achieve tis, the position of the major deflector in a subfield at a certain instant is read and determined, and the major deflector is shifted to and fixed at a required position. Thereafter, the minor deflector is feedback-controlled to control the deflection width thereof in response to the movements of the stage as well as to control the drawing of patterns in the subfield. Namely, once the position of the major deflector is read at a certain moment, a coordinate X1 (a major deflector shifting vector) according to which the major deflector is to be adjusted is determined as mentioned above.
To correct a rotation error of the major deflector as well as a deflection sensitivity error of a major amplifier, the main deflection shifting vector X1 is passed through a major deflector correction operating circuit 14 to provide a data signal to a major deflector DAC (Digital Analog Converter) 15.
A set strobe is applied to the major deflector DAC 15 at a certain time after the major deflector coordinate of the selected subfield is read out of the memory 1. This is because the major deflector needs a certain settling time after it is shifted to the specified coordinate position. To actually operate the major deflector to expose a selected one of the subfields S1 to Sn in the main field M and draw patterns in the selected subfield after the settling time, a latency time generator 21 is provided. The output signal of the latency time generator 21 activates the pattern generator 4 for the subfield (S1 - Sn).
The latency time varies depending on the coil and amplifier of the major deflector, and is usually 1 msec for a jump distance of 2 mm.
After the major deflector is oriented to the main field M, the major deflector is successively fixed to particular coordinates of the subfields S1 to Sn, and at each time, the pattern generator is operated after the latency time to deflect charged particle beams. If gain data and rotation data for the minor deflector are needed, these data are read from the correction data memory 13 and stored in a minor deflector correction operating circuit 26, in which an output of the subfield pattern generator 4 is corrected. The circuit 26 provides minor deflector adjusting data to a minor deflector DAC 27. ("Minor" and "sub-" are used synonymously herein.)
A subtracter 22 calculates a difference between a position of the stage 5 read at read timing and stored in the stage read register 8, and a count of the laser counter representing a present position of the stage 5. The subtracter 22 provides the difference to a minor deflector feedback correction operating circuit 23, which provides its data to a minor deflector feedback DAC 24.
A deflection signal for each shot provided by the pattern generator 4 for the minor deflector and the feedback signal are converted into analog signals by DACs 27 and 24, respectively. The reason for this is because, for the deflection signal, intervals of strobe pulses fluctuate depending on exposure clocks for respective shots, while the feedback signal depends on up/down pulses (a pulse interval is 60 nsec [15 MHz] when the stage moves at a speed of 70 mm/sec at an accuracy of .lambda./120=0.0051 .mu.m) of a laser interferometer of the stage. It is difficult, therefore, to form synchronous signals for the deflection signal and feedback signal. It is not necessary to synchronize digital pulses and no digital operational errors occur if the signals are converted into analog signals and then added to each other.
According to the conventional charged particle beam exposure system mentioned above, the major deflector is deflected from one subfield S1 in the main field M to an adjacent subfield S2 in the same main field with a certain settling time T1 as described before. When the major deflector is going to be shifted from the subfield S1 to the adjacent subfield S2, or to a certain subfield of an adjacent main field, beam emission is usually stopped and deflector shifting is not carried out, if a value of a vector sum of a distance between a target stage position and a present stage position and an output value of the major deflector which is the main deflector coodinate (X.sub.1, Y.sub.1) of the subfield. (i.e., a major delector shifting vector [X.sub.1, Y.sub.1 ] finally output to the major deflector DAC 15.) is longer than a drawable area (for example, 2 mm). After the stage moves to shorten the distance and enter the drawable area, the major deflector is shifted to the center of an edge subfield (for example, a lower left subfield) of the adjacent main field. To shift the major deflector, a shifting vector X1 of the major deflector is calculated as mentioned before. This vector X1 corresponds to a distance between points P and Q shown in FIG. 4.
An actual movement of the major deflector from the point P to the point Q follows a dashed line K or K' shown in FIG. 4, and it usually requires a latency time of 1 msec for a jump of 2 mm. This latency time is dead time because beam exposure must be stopped during the latency time. The latency time is, therefore, a bottleneck for improving the throughput of the exposure system. During the latency time of 1 msec, the stage is continuously moving. To carry out exposure after the latency time, it is necessary, therefore, to calculate a correction value according to a difference between a target position and a present position of the stage, and adjust the major deflector according to the correction value. This correction involves another latency time and positional deviation. This means that the correction process must be repeated endlessly, thereby complicating the whole process.
For example, it is supposed in the above-mentioned conventional system that the width of one main field is 2 mm, a jumping distance of the major deflector is 2 mm at the maximum, a latency time after a jump of the major deflector is 1 msec, and a possible range of feedback control of the minor deflector for compensating movements of the stage is .+-.10 .mu.m, i.e., a band of 20 .mu.m.
When the stage is moving at about 5 mm/sec or slower for a low-speed exposure, the stage moves only 5 .mu.m during the latency time of 1 msec, so that, by adding 10 .mu.m to the minor deflector feedback circuit in advance, the minor deflector feedback control will be effective for a range of 15 .mu.m. In this case, even if the pattern generator starts after the latency time of 1 msec, there is still a time margin of 3 msec in exposing a subfield, so that the subfield may be sufficiently exposed.
When the stage is moving at 50 mm/sec for faster exposure, a problem occurs. Since the allowable width of the feedback control for a subfield is 20 .mu.m, the stage stays only for 400 .mu.sec in the minor deflector feedback range. It is necessary, therefore, to always judge whether or not the position of the stage overflows the subfield feedback range. If it overflows, the major deflector must follow the stage position, or the pattern generator must be stopped.
No overflow may occur on the minor deflector feedback control range if the pattern generator is not expected to operate, or during a waiting time for a major deflector jump. Once the overflow occurs just after the start of exposure, or after several exposures, the exposure must be stopped. Namely, the exposure is done irregularly, if an overflow occurs. This sort of irregular control cannot improve the throughput of the exposure system and cannot draw uniform patterns because it causes dislocation of patterns and changes in exposure conditions.